1. Field of the Invention
The present invention relates to a transparent conductive film having high optical transmittance, and the transparent conductive film may be widely used in a transparent electrode of a touch panel, a liquid crystal display (LCD), an electronic-paper (E-Paper), a solar cell, an electronic induction display (EL), various soft electronic products having flexibility, and various electrical fields.
The present invention also relates to a method for preparing the above-mentioned transparent conductive film.
2. Description of the Prior Art
In recent years, a notebook computer and a mobile phone attached with a touch panel are getting more and more popular, and the industry also raises more requirements on a touch panel having excellent electrical and optical properties. A transparent conductive film is a key component of a touch panel, and a current manner for preparing a transparent conductive film is mainly a dry process, such as a vacuum evaporation method, a sputtering method, ion implantation, and a chemical vapor deposition method, but the dry process has the following problems.
(1) During a machining process and rolling sample storage, the surface of a thin film is easily ruptured, and the surface impedance is increased (such as a process disclosed in Japanese Patent Publication No. 2006-302562).
(2) The machine used is expensive and results in an increase of the cost during mass production (such as a process disclosed in Japanese Patent Publication No. 2008-059928).
However, a wet coating process for producing a transparent conductive film may solve the above-mentioned problems, and compared with the dry process, the production efficiency of the wet coating processes higher and the needed device is relatively simple. The conventional wet coating process is mainly to formulate a conductive material into a solution, and then coat the solution on a substrate, to form a conductive film.
Generally, an ordinary conductive material used in wet coating process includes graphene, poly(3,4-dialkoxythiophene), and carbon nanotube, but the above-mentioned materials all have a color, are uneasily manufactured into a conductive film having transparency, and are thus limited in applications. Therefore, it is an important topic to improve the transparency thereof.
Therefore, recently, the development of a wet process to prepare a transparent conductive film is getting popular, as described in U.S. Pat. No. 7,449,133, Japanese Patent Publication No. H01-313521, Japanese Patent Publication No. 2002-193972, Japanese Patent Publication No. 2003-286336, U.S. Pat. No. 7,378,040, Japanese Patent Publication No. 2005-281704, U.S. Pat. No. 7,060,241, U.S. Pat. No. 7,172,817 U.S. Pat. No. 7,261,852, and U.S. Pat. No. 7,459,121. These patent documents are briefly classified and illustrated as follows.
U.S. Pat. No. 7,449,133 discloses an innovative conductive material of graphene, which is modified with a molecular segment having a conductive functional group, so as to increase the number of conductive paths between graphenes, thereby improving the conductivity and the optical transmittance. However, such modification method is complicated, low yielding, and costly.
Japanese Patent Publication No. H01-313521 discloses that poly(3,4-dialkoxythiophene), which is obtained by oxidation and polymerization of 3,4-dialkoxythiophene in the presence of a polyanion, reacts with a polyanion to form a conductive polymer. Japanese Patent Publication No. 2002-193972 and Japanese Patent Publication No. 2003-286336 further disclose that higher optical transmittance and lower surface resistance (i.e., higher conductivity) can be achieved by modifying the conductive polymer, however, when the conductive polymer is used to produce a conductive thin film, the humidity and heat durability of the film is not good.
U.S. Pat. No. 7,378,040 discloses that a fluoropolymer or a fluoro-containing monomer is added to a carbon nanotube as a binder. By adding this kind of polymer or monomer, several properties such as transmittance, electrical conductivity, and mechanical strengths can be improved. However, fluoropolymer is not cost effective and will adversely affect the environment.
Japanese Patent Publication No. 2005-281704 discloses that the weather durability of a conductive polymer can be increased by adding various binders. Nevertheless, the conductivity of the conductive polymer would be decreased.
U.S. Pat. No. 7,060,241 discloses adjusting the outer diameter of a carbon nanotube to be smaller than 3.5 nm, to obtain a carbon nanotube that has high transmittance. However, it is difficult to control the outer diameter of a carbon nanotube within a certain range when the carbon nanotube is growing and being screened.
U.S. Pat. No. 7,172,817 discloses that when a conductive particle is changed from a circular shape to a flat shape, the probability of contact between the particles is increased, and the transmittance is also increased. However, the preparation of such conducive particles yields low and is costly.
U.S. Pat. No. 7,261,852 discloses that a coating containing a carbon nanotube is filtered with a filter to deposit the carbon nanotube on the filter, so that a thick carbon nanotube bulk may be deposited by the method, and a binder may also be filtered, to achieve a high electrical conductivity. However, this method may result in uneven film thickness, an unstable resistance value of a subsequent thin film, and a low visible light transmittance.
U.S. Pat. No. 7,459,121 discloses that a carbon nanotube conductive film is prepared by dip coating, and meanwhile a roll-to-roll process may be accomplished with the same coating device. However, the coating concentration in the dip coating process is extremely unstable, and a prepared thin film could not have uniform surface resistance values.
As described above, the conductive film prepared by the wet process in the prior art cannot acquire characteristics of a uniform resistance value and a high visible light transmittance simultaneously, or the preparation process is excessively complicated. Therefore, in order to solve the above-mentioned problems, the industry needs a method for preparing a conductive film, which not only satisfies requirements on various characteristics such as the electrical property, mechanical strength, and weather durability of the conductive film in the industry, but also increases the optical transmittance and simplifies the steps of the process.